Document Alert Banners

[msayoung]

This is partially done.

• [x] Technical docs
• [ ] Content designer docs
• [ ] Background

[finnlewis]

Not heard from @danchamp for a little while.

Passing to @andybroomfield to review and confirm what we need.

[danchamp]

@finnlewis @andybroomfield There is a branch with limited progress to date: https://github.com/localgovdrupal/docs/tree/feature/52-alert-banners

I'm struggling for time to pick this up just now.